1. Field of the Invention
This invention is directed to wellbore cementing operations and apparatuses and methods for use in such operations and particularly to devices for alleviating hydraulic locks encountered in a wellbore and to cementing tools called stage tools and methods of their use.
2. Description of Prior Art
A drilled wellbore hole is prepared for oil or gas production by cementing casing, liners or similar conduit strings in the wellbore. Cementing is the process of mixing a composition including cement and water and pumping the resulting slurry down through the well casing and into the annulus between the casing and the wellbore. Cementing provides protection from the intermixing of the contents of various production zones which could result in undesirable contamination of produced oil or gas or in contamination of the producing strata.
In the early days of the oil field industry, the shallower wells allowed cementation to be accomplished by pumping a cement slurry down the well casing, out the casing bottom, and back up the annular space between the bore hole and casing. As wells were drilled deeper, the cementing process was accomplished in two or even three stages. Cementing tools, stage tools, or ported collars equipped with internal valving, were needed for multi-stage cementation.
Typically, the internal valving of cementing tools, or stage tools, consist of one or more sliding sleeve valves for the opening and closing of the cement ports before and after a cement slurry has passed through the ports. A variety of plugs are used to aid multi-stage cementing to open and close the correct sleeve valve at the correct time.
Problems have been encountered when two sleeve valves are employed to open and close the cement ports. The sleeve valves are shear-pinned in an upper position with the lowermost sleeve sealing the ports closed for running in the wellbore hole. When stage cementing is desired, an opening plug is moved, or dropped and gravitated, to seat and seal off the lowermost sleeve. Pressure applied at the surface applies enough downward force on the plug and seat arrangement to break the shear pins and shift the lower sleeve valve down, thus opening ports which allow cementing solutions or slurries to flow down the interior of the casing and then through the ports into the annulus between the exterior of the casing and the interior of the wellbore. Cement is pumped down the casing, through the ports and back up the annulus.
As the tail end of the cement slurry is pumped down the casing, a second plug often called a "closing plug" is placed into the casing behind the cement. This plug moves down to seat and seal off the uppermost sleeve valve until sufficient surface casing pressure is applied to break the shear pins holding the sleeve. The upper sleeve and plug shift downward to cover and seal off the ports so that no more solution or slurry passes either into the annulus or back from the annulus. An engaging mechanism can be used to lock the closing sleeve in position.
A problem has been encountered in this operation due to the creation of a hydraulic lock when a seal is established across the ports. When a portion of the solution or slurry ahead of the closing plug is pushed downwardly and the ports close off, this small portion of fluid becomes trapped between the plugs within the stage tool and can flow nowhere. The nearly incompressibile nature of the trapped material does not allow the upper sleeve valve to travel sufficiently downward to engage a positive locking mechanism to prevent a reopening of the ports. If the engaging mechanism does not engage on the upper sleeve valve, internal casing pressure must be held until the cement sets.
In accordance with .sctn.1.56 of 37 C.F.R. the following references are disclosed and copies thereof are submitted herewith:
(a) U.S. Pat. Nos. 2,602,510; 2,928,470; 3,811,500; 3,824,905; 3,948,322; 3,768,556; and 4,487,263; PA1 (b) publications including: Dowell Schlumberger, CEMENTING TECHNOLOGY, (1984) "Primary Placement Techniques", Chapter 10, pp. 1-20; Chapter 13, "Cementing Equipment", pp. 11-12; Halliburton Services Sales & Service Catalog 43, COMPOSITE CATALOG, pp. 2440-2451 (1986-1987); Weatherford Cementing Program, especially pp. 36-37 (1986)
U.S. Pat. Nos. 2,928,470 and 2,602,510 disclose a device used in a two-step operation for closing off cementing ports that allows a lower sleeve valve to shift in unison with an upper sleeve valve the instant before a hydraulic lock is effected. This is accomplished by introducing a third shear means to release the lower sleeve valve as the upper sleeve valve lands on its upper side. This configuration has the inherent disadvantage of prematurely shearing the third shear means (and thus fouling it) when the lower sleeve valve impacts with too much force while opening the ports or by excessively high cement pump pressures exerting a sufficient downward force before closing is desired. U.S. Pat. No. 2,602,510 discloses the use of bleeder holes which provide fluid communication between the casing interior and the annulus. Since the holes are relatively small and are always open, they can plug up with cementing particles or well mud particles. Also, because of the design of the ported cementing apparatus and the disposition of its seals, the holes must be small (and hence pluggable), otherwise the seals which are required for proper operation of the tool will not be able to properly seal off the holes. U.S. Pat. No. 2,602,510 discloses a device which has seals which can be permitted to move past the ports. Because of the flow of fluid under pressure through the ports and its impact on a seal, the seal can be damaged in the areas adjacent the ports. This same problem can happen in the prior art devices of U.S. Pat. No. 3,811,500.
U.S. Pat. Nos. 3,811,500 and 3,842,905 disclose a device which uses an opening plug to shift a lower sleeve valve open. As the upper sleeve valve slides to cover and seal flow ports, the closing plug, used to shift the upper sleeve valve closed, imposes a downward force on a rod extending through the opening plug which breaks shear pins holding the rod in place and opens a passage through the opening plug for trapped fluid to exit. This configuration is pressure sensitive to excessively high cement pump pressure which can break the shear pins and cause undesirable premature activation; i.e. the rod is pushed out during the cementing operation rather than at its completion. Also, there is no guarantee the mechanism will be aligned correctly upon the seating of the opening plug, due to the loose fitting characteristics of such a plug and the requirement that the plug go down the casing in a properly aligned configuration. If the plug becomes misaligned the device will not work properly. Because of the sensitivity of the shear pin used to hold the rod in place, it is difficult if not impossible to use a hammer means such as a drill pipe joint to jar a stuck plug--since such jarring will cause premature release of the rod or the plug may become damaged.
U.S. Pat. Nos. 3,768,556 and 3,948,322 disclose a device similar to that of U.S. Pat. No. 2,928,470, but with a metal locking device for holding the various elements of the tool in fixed relation to each other, including holding the opening and closing plugs in fixed relation to each other so that the closing plug at some point no longer pushes down on fluid trapped beneath it, eliminating further hydraulic locking effects.
U.S. Pat. No. 4,487,263 discloses a device which has small apertures which permit flow from the interior of the device, where cementing fluid may be trapped, into a chamber in the device. These holes can be plugged up by cementing fluid or mud and the chamber may not be large enough to hold all the fluid. If the chamber has filled and there is still more fluid trapped between the plugs, there can still be an unwanted hydraulic lock.
The listed publications generally describe well cementing operations and stage tools.
There has long been a need for an effective and efficient cementing stage tool and methods for its use. There has long been a need for a device for alleviating hydraulic locking in a wellbore. There has long been a need for a stage tool which does not activate prematurely, which has ports not subject to unwanted plugging, and which does not require complex engaging mechanisms. Also there has long been a need for such a tool and device which do not damage seals used therein. The present invention recognizes, addresses, and satisfies these long-felt needs.